An unexplained epidemic of chronic kidney disease has claimed the lives of at least 20,000 people in Central America in the past two decades. The illness is not related to diabetes or hypertension—the most common drivers of kidney disease in the United States. BU researchers have been working since 2009 to uncover the causes of this epidemic.

A new study led by School of Public Health researchers suggesting that heat stress or other occupational factors may be playing a role in the high rates of chronic kidney disease disproportionately affecting young, male agricultural workers in northwestern Nicaragua is a step toward identifying the factors responsible. The researchers found that sugarcane workers in the region have experienced a decline in kidney function during the harvest, with field-workers at greatest risk. The study was published online in January in the International Journal of Occupational and Environmental Health.

The research team recruited 284 Nicaraguan sugarcane workers, mostly men and with a mean age of 33.6, performing seven different tasks. They measured urine albumin and serum creatinine and estimated glomerular filtration rate—markers of kidney function—both before and toward the end of the harvest season. Workers were asked about the quantity of water and electrolyte solution packets they consumed during a typical workday.

The researchers found that kidney function declined during the six-month harvest season and varied by job category, and that workers with longer employment duration had worse kidney function. Over the harvest season, the decline in kidney function was greatest among seed cutters, irrigators, and cane cutters—jobs that require strenuous labor in a hot and humid environment. The results provide evidence that one or more risk factors for the disease are occupational, they said.

The research team said that inadequate hydration also might play a role. While there was no effect of water or electrolyte solution consumption on kidney function among all workers, the study did find a protective effect of electrolyte solution consumption among the cane cutters—workers who have the heaviest workload.

The results are “consistent with the hypothesis that heat stress and dehydration may play a role in kidney disease,” they said, and that further study is needed to directly measure workers’ exposures to heat and dehydration, rather than using job categories as a surrogate exposure measure.

The study did not find support for the hypothesis that agricultural chemicals are a causal agent for kidney disease. Instead, among field-workers, agrichemical applicators, who have the most direct contact with chemicals, were found to experience the smallest decline in kidney function.

“Agrichemicals are used extensively in the region and widely believed among community members as the cause of the epidemic,” according to the study. The authors noted that in September 2013, El Salvador’s National Assembly voted to ban 53 agrichemicals because of the perceived link to chronic kidney disease. “Agrichemicals, therefore, remain an important hypothesis and warrant further study,” the authors wrote.

The research team is planning a new study to quantify exposure to heat and agrichemicals among manual laborers, including sugarcane workers, in Central America, and to determine whether those exposures are associated with kidney injury.

Among the SPH study authors were Madeleine Scammell and Michael McClean, associate professors of environmental health, Daniel Brooks, an associate professor of epidemiology, and environmental health doctoral candidate Rebecca Laws. Researchers from Tufts Medical Center, VA New York Harbor Healthcare System and Department of Medicine, Universidad Autónoma de Madrid, and Yale University contributed. Funding for the study was provided by the Compliance Advisor/Ombudsman (CAO) of the World Bank Group and the Comite Nacional de Productores de Azucar.

A version of this story originally appeared on the School of Public Health website.